A Guide to Purchasing an Industrial Reverse Osmosis (RO) System

A Guide to Purchasing an Industrial Reverse Osmosis (RO) System

Choosing the right industrial reverse osmosis (RO) system isn’t just about picking a model and hoping it works. It’s about finding a solution that aligns with your site, your specs, and your long-term goals.

That’s why we created The Ultimate Buyer’s Guide: Reverse Osmosis— a comprehensive, easy-to-follow resource built specifically for industrial water users.

Whether you’re new to RO systems or looking to upgrade an existing one, this guide walks you through everything you need to know to make a smart, informed decision.

The selection process is broken down into 5 steps:

1. Desired Water Quality and Quantity
2. Site Specifics
3. Narrowing Down Available Options
4. Ongoing Maintenance
5. What Does an RO Cost?

Step 1: Water Quality & Quantity

What Is Your Current and Future Demand for RO Product Water?

RO systems are rated on permeate flow, not feed flow. In the US, the units are either GPM (gallons per minute) or GPD (gallons per day). So, when we talk about a 100 GPM RO system, that is referring to the amount of permeate (or product water) that the RO system produces, not the feedwater flow.

To convert GPM to GPD, multiply GPM x 1440 (a 100 GPM RO = 144,000 GPD). The feed and concentrate flow depend on what recovery rate the RO is operating at and this is typically dictated by the feedwater quality and determined by the engineer designing the system.

To determine the correct RO size, the first step is to understand how many gallons of RO permeate water you will require each day and the expected demand throughout the day. What we are looking for is any large spikes in demand throughout the day to make sure that the RO system, storage tank, and distribution pump are sized properly.

Sizing a Reverse Osmosis System

Once you know how much water you require per day and the expected peak in demand, then we can select an RO system, combined with a RO permeate storage tank, that can accommodate this demand. Ideally, you want a RO permeate storage tank that is large enough so your RO system is not constantly cycling on and off and can hold enough treated water to give you a good buffer during times of high demand.

When an engineer designs the RO system, they base the expected permeate flow on a set feedwater temperature, usually 70°F (21°C). Water temperature has a big impact on how much water passes through an RO membrane. For every 1°F drop in water temperature, permeate flow decreases by 1.5%. Why? Colder water is denser than warmer water, and it takes more energy to push dense water through the RO membranes. So, if you install a 100 GPM RO system and it is designed with a feedwater temperature of 70°F and the feedwater temperature drops to 60°F in winter, then the permeate flow will be closer to 85 GPM. It’s important to consider the expected lows in feedwater temperature throughout the season. This is especially true if your feedwater comes from a surface water source.

Another factor affecting permeate flow is that RO membranes will inevitably become fouled and/or scaled through normal operation (no matter how good the pretreatment is), and this will decrease permeate flow. The amount of decreased flow will depend on the severity of fouling/scaling.

Although proactive RO membrane cleaning can help return membranes to near-new RO permeate production, they will continually decline as they age and eventually need to be replaced (due to poor permeate production, loss of salt rejection, etc.)

In summary, if you need 140,000 GPD of RO permeate for your production needs, then do not purchase a 100 GPM RO. It’s better to select a larger 125 or 150 GPM RO that can handle feedwater temperature and salt concentration fluctuations, as well as provide some room for future growth in water demand.

Having a large RO permeate storage tank has its advantages as well. For example, you could run the RO at night to fill up the storage tank to save on electrical cost.

Also, whether you clean membranes on-site with a Clean-in-Place (CIP) system or send them to a third party for offsite cleaning, cleaning and/or moving membranes in/out takes a lot of time. A large storage tank will allow you to operate through these infrequent events.

What is the Water Quality that You Require at the Point of Use for Your Application?

This question, combined with an analysis of your feedwater, will determine fi the RO needs to be a single-pass or double-pass design and what kind of pre- and post-treatment is required.

A single-pass RO system will typically remove up to 99% of salts and contaminants. A double pass RO system will then remove greater than 90% of the remaining 1% that made it through the first pass, resulting in higher quality permeate. Another way to think about it is that with a double pass RO system, the feedwater goes through one RO system, and the resulting permeate water is fed into another RO system, and that permeate from the ‘second pass’ is your final permeate from the double pass RO system.

For example, if your feedwater is 500 ppm TDS, a single pass RO system will result in permeate water quality of approximately 5 ppm, whereas a double pass RO system will result in permeate water of approximately 0.1 to 0.05 ppm.

Your desired water quality at the point of use will also determine if you require ion exchange mixed beds to polish the RO permeate water.

If you do require mixed beds to polish the RO permeate, a double-pass RO can extend the life of mixed beds. This is because a double pass RO produces permeate water with much less ionic contaminants and as a result, the mixed bed polishers last much longer and require fewer exchanges. This results in worthwhile cost savings over time.

Step 2: Site Specifics

What are the Feedwater Characteristics?

Feedwater to an RO system typically falls into 5 categories:

• Municipal water
• Untreated well water
• Untreated surface water (lake, stream, etc)
• Sea water
• Wastewater

Feedwater characteristics are arguably the most important factor in RO design. A designer will obtain a feedwater sample and run a full laboratory analysis to determine the feedwater composition. Based on these results, the designer will choose the appropriate pretreatment equipment, flux rate, RO membrane type, feedwater spacer size, stage, pass, array arrangements, pump size, and a host of other decisions will be made based on the feedwater analysis.

Choosing the right equipment is accomplished with the help of RO design software. However, the real value comes from having an experienced engineer take a holistic view of the entire system and consider site-specific issues along with future issues that might be around the corner.

The feedwater analysis is critical for system design. It’s a great idea to obtain another water analysis at a later time and compare results. Feedwater composition is constantly changing due to droughts, upstream water treatment methods, alternating water sources, and other factors. Also, it’s important to follow best practices while collecting a water sample so as not to base the system design on erroneous results. Also, a designer will need to know the feedwater flow rate, pressure, and pipe size available to the RO system during the design phase.

Are You Able to Discharge Concentrated Brine at Your Facility?

An RO system produces two streams of water. One stream is the permeate, and the other stream is the concentrate (also called reject or brine). The concentrate stream carries all of the contaminants that the RO membranes reject, and this waste stream typically goes to drain, unless it can be further treated or recycled.

It’s important to understand that the concentrated salts in the concentrate stream may pose a problem with discharge limitations and/or extreme scale formation in the concentrate discharge line. As more water is recovered as permeate, more salts and other contaminants rejected by the RO membranes are collected in the concentrate stream.

Below is a chart that shows different RO system recovery rates and their associated concentration factors. In this example, we are using a feedwater flow rate of 10 GPM and feedwater quality consisting of 500 ppm of dissolved solids.

For example, fi the feedwater TDS to the RO system is 500 ppm and the system is designed to run at 80% recovery, then the expected salt concentration in the concentrate stream would be 500 ppm x5= 2,500 ppm. If you increased the recovery rate to 90%, then the concentrate stream would double in concentration to 5,000 ppm.

If you are unable to discharge brine at your facility, then you will either need to consider treating the concentrate stream and reusing the water so it doesn’t go to drain (not a trivial undertaking), haul off the concentrate by truck to a facility that can accept the brine, or using a different method, such as an offsite ion exchange service, to demineralize your feed water.

None of these options are as economical as using an RO system and sending concentrate to drain, but sometimes you have no other option. The important thing is to check with your municipality and determine if you can discharge concentrated brine to drain. If you need help, Puretec has a list of consultants that can help you through this process.

Installation Considerations

• All utilities such as water, power, and sewer need to be plumbed to the RO system.
• It’s important to consider that an RO system is prone to leaks. If the RO system is going to be inside, then berms and drainage should be installed to handle the water drainage during membrane changeouts and inevitable leaks.

Space Considerations

• It’s important to measure how much space you need for the RO system and ancillary equipment (pre and post-treatment, the permeate storage tank, etc.). You also need to consider service access for removing and adding RO membranes to the pressure vessels, accessing pumps and filters for replacement or changeouts, etc. And lastly, you need to make sure your installation meets all NEC code requirements.
• If the RO system is going to be outside, it’s important to protect the RO system from outside elements or enclose it in a container or mobile trailer.

Step 3: Narrowing Down Available Options

Do You Require Any Special Material Considerations or Approvals?

Standard RO designs are a great fit for most applications, typically cost less, and offer faster delivery. However, there may be a need for customization. These might include specific instrumentation, redundant components, piping or frame material, frame colors, and so forth. Likewise, some systems need to be validated and others need approved PE stamped drawings, seismic calculations, and special electrical requirements such as explosion proofing.

Do You Want to Add a Clean in Place (CIP) System for Your RO Membranes or Have a Service Company Clean Them Offsite?

The RO membranes will need to be cleaned periodically. A CIP system is typically installed adjacent to the RO system and allows you to clean the RO membranes in place. The primary advantages of a CIP include the ability to clean onsite. The disadvantages include the increased cost and space requirements of the CIP system, handling and storing low and high pH chemicals used for cleaning and disposing of those chemicals onsite. A proper CIP cleaning takes as much as 12 hours, and many facilities do not have the time or expertise to accommodate this.

The alternative to a CIP is to have a service company come to your facility, remove the RO membranes, and take them to a cleaning facility. After cleaning, they are returned to your facility and installed in the RO system. A spare set of RO membranes is installed in the RO to allow uninterrupted production while your fouled membranes are being cleaned. Either the service company provides these membranes, or the customer has a spare set onsite (which is good insurance).

Offsite cleaning has its advantages. Because each membrane is individually weighed and inspected (broken anti-telescoping devices, fiberglass casing cracks, brine seals, product tubes, etc.). Each cleaning vessel holds only three membranes versus the four to six membranes typically found in a vessel during a CIP process. Each membrane’s performance is tested before and after a cleaning to ensure that the process was successful and/or identify and replace those membranes that are damaged or irreversibly fouled.

Do You Want the Ability to Monitor the RO System Remotely?

Adding sensors for remote monitoring and pairing them with software that collects data, normalizes trends, and sends proactive system alerts is a worthwhile investment. Some customers want their RO system to communicate with their building management system, such as a Building Automation and Control Network. All of this is possible, but it’s important to specify this early in the buying process.

Step 4: Ongoing Maintenance

An RO system will provide many years of dependable results if someone pays attention to it and is proactively maintaining the unit. Ideally, an operator is collecting data on the RO system daily and physically inspecting the pretreatment equipment, RO system, and post-treatment equipment, if any.

The operator also needs to be able to interpret this data and take proactive steps before small issues turn into big issues. As any experienced RO operator will attest, small seemingly insignificant things quickly add up and become serious problems.

Just like IT or HVAC infrastructure, the ideal outcome is to have an experienced professional onsite who knows RO systems and has decades of experience maintaining and troubleshooting them. This is not always possible, and a company needs to consider its resources.

RO maintenance is ultimately the experience and resources they have access to. RO system maintenance and troubleshooting is not an easy skill to obtain, and you need to consider the cost of having your water system shutdown versus investing in an effective maintenance program. A good service provider will also offer spare membranes and many stock parts in case of emergencies.

A good service provider should have:

• Experienced technicians
• The ability to properly collect, interpret and report RO data
• The ability to clean RO membranes when required
• 24-hour emergency support.

Reverse Osmosis data collection is important in assessing membrane conditions and addressing potential problems.

Another important consideration is the trend towards operating RO systems at a high recovery and the effect that has on your sewer line.

As RO systems are being pushed to their upper recovery limits, the reject stream is so concentrated that the salts can fall out of solution soon after entering the drain line, precipitate, and form a scale.

We’ve seen many customers with drain lines completely scaled up and unable to accept the reject stream, resulting in a system shutdown until the pipe can be cleaned, which can be difficult, depending on the precipitant. It’s a good idea to occasionally inspect the sewer line and take proactive steps to clean it.

Step 5: What Does the RO Cost?

Understanding the Big Picture

Unless your only need is to replace an existing RO skid with similar performance capabilities, you are actually purchasing a system, not a component. ROs are excellent at removing the bulk of dissolved metals, organic material, and bacteria, but they don’t operate without proper pretreatment, and the water quality produced by an RO may or may not meet your final water quality requirements.

Pretreatment Considerations are Driven by the Type of Feedwater Being Treated

• A surface water source may have low hardness and not present much of a challenge for scaling in the RO concentrate, but it may be high in total suspended solids (TSS) and organics, requiring robust media filtration and granular activated carbon (GAC) to remove particles and total organic compounds (TOC).
• A well water source may have low TSS and low turbidity but have an elevated conductivity value with high hardness. In this case, less filtration may be required, but either softening or chemical treatment may be required to reduce scaling potential in the RO concentrate.
• A wastewater source will almost always be the most difficult source to treat, requiring advanced pretreatment processes and may include ultra-filtration, micro-filtration, in addition to media filtration, carbon filtration and chemical treatment.

And there are always several variables to consider when determining what components are necessary in a properly designed system. The goal is to balance initial capital costs with long-term maintenance costs.

• Should you buy the lowest cost RO you can source? Maybe, but lower cost often means lower quality, and lower quality often means more diligent upkeep and more frequent repair and replacement. Are you staffed for that?
• Should you skimp on pretreatment filtration to save upfront costs, only to suffer poor RO operation soon after start-up, months and months of frequent RO membrane cleanings, and premature RO membrane replacement?
• Will you need to further treat the product water RO with an ion exchange process to meet, for example, boiler feedwater requirements? Then does it make sense to purchase a single-pass RO system followed by service ion exchange and experience high long-term ion exchange service costs? Or would it be more cost-effective to purchase a more expensive two-pass RO system that would drastically reduce long-term ion exchange service costs?

A reputable company with experienced applications engineers should be considered to review any RO treatment needs. They use industry-developed tools and can define the most efficient and cost-effective processes to reach the final quality goal of any system and optimize upfront and long-term operating costs based on your operational expectations.

Standard Manufactured ROs Versus Modified or Customized ROs

All RO equipment manufacturers develop standard RO designs. The development of these designs is based on industry knowledge and process experience.

Some ROs are a good fit in light industry applications but lack the robust needs of say a refinery. An RO designed for heavy industry applications may include features or components that are overkill for a small electro-plating service.

Standard ROs offer a proven design with industry-standard components and materials of construction. Since they are predesigned, the cost of engineering has been distributed across a vast family of ROs and the cost is always lower than starting from scratch. And for those same reasons, standard ROs can be delivered much faster than custom designs.

Modified or customized ROs are usually standard RO systems that require small to major adjustments. For example, a customer may have a facility with instrumentation standardized across the facility and desired that same instrumentation be provided on any new equipment installed in the facility.

An RO manufacturer may then need to incorporate this customer-defined instrumentation into the RO design. This might require modifying panel drawings used in manufacturing by adjusting the instrument cut-out dimensions or, in some cases, using a large panel to accept the added depth of the custom instrument. Now that we have a large panel, does it fit on the standard mechanical frame or are weldment or do we need to redesign the frame?

Other examples of customization include:

• Special piping materials – for example, due to high chloride corrosion concerns in wastewater applications
• Larger high-pressure pumps – for example, due to higher conductivity feedwater
• Footprint modifications – for example, due to site-specific space restrictions

These adjustments take additional time to engineer, design and review. They create changes in the manufacturing process that typically increase labor costs. And this all takes time to complete, increasing lead-time. A customer should strongly consider standard RO designs from experienced RO manufacturers as a starting point. Standard ROs are not meant to fit all needs, but rather to fit most needs in a cost-effective manner.

Installation

Installation costs can vary from a fraction of the capital cost of the system to more than the cost of the system. This is driven by several factors. A small system may be easy for a customer to self-install. A mid-sized system with general complexity may be perfect for an equipment vendor to install. A customized large system may be best installed by qualified mechanical and electrical contractors and an engineer to ensure complex building codes are met.

Very few customers have staff with the time and ability to properly install components they have little experience with. A good starting point is to discuss this with your system provider. Many providers can install the equipment they provide in a cost-effective manner. Providers also know their limitations and can offer contractor recommendations and even manage those contractors on behalf of the customer.

Summary

We’ve installed many RO systems over the years. Ultimately, every system is site specific and it’s critical to thoroughly go through this discovery phase to make sure you end up with the best RO system for your application. As you consider options, keep in mind the real cost of having your water system fail and shut down production.

RO Discovery Checklist

• What is your current and future demand for RO product water in GPM or GPD?
• What is the water quality that you require at the point of use in your facility?
• What is the feedwater source to the RO system?
• Are you able to discharge concentrated brine at your facility?
• What are your available utilities and sizing?
• What space limitations do you have?
• Do you require any special material requirements or approvals?
• Do you want to add a clean-in-place system for your RO membranes or have a service company clean them offsite?
• Do you want the ability to monitor the RO system remotely or tie it into an on-site communication protocol?

Download our Ultimate Buyer’s Guide: Reverse Osmosis to help you find a system that’s right for you.